The present invention relates to a pressure relief valve designed for the vent line of a hydrocarbon storage facility.
It is current practice in the petroleum industry to have vent lines running from underground storage tanks in a service station to an above ground location so that pressurized vapors in the tanks may be released into the atmosphere. Also, when gasoline is being pumped from an underground tank, a vacuum normally develops and the vent line allows inbreathing of air into the tank. Recent Environmental Protection Agency (EPA) regulations have required the recovery of gasoline vapors previously vented to the atmosphere, and this has required a modification of the venting system. These environmental regulations will probably require a restrictive device on the end of a vent line to reduce the escape of hydrocarbon vapors into the air.
Several types of pressure/vacuum valves are currently available on the market to perform this function. Most of these devices employ springs to control the relief pressures. These valves are often unreliable because the spring forces tend to vary with age, weather conditions, and other factors. Also the possibility exists for the components to become frozen together because of moisture trapped therein during subfreezing temperatures. Further, in normal usage these devices often require some movement of internal components for a pressure release to occur. Also, these types of valves tend to be fairly complicated and quite expensive.
Another approach to this problem is disclosed in U.S. Pat. No. 4,007,759, for Pressure Stabilizer For Storage Facilities and assigned to Sun Oil Company of Pennsylvania. That invention utilizes a spherically shaped element placed above an aperture in such a manner as to allow normal inbreathing and outbreathing. In the event the pressure build up in the system is too great, the ball is lifted off the seat to allow additional outbreathing. The apparatus disclosed in that application is quite expensive to manufacture because of the requirement for a spherical relief element, and further would probably not meet certain Underwriters Laboratories requirements because of its mode of construction.